To be clear , the wear in the photo above is not "natural " . just made to look that way ./( sand paper , and instant coffee stain in appropriate areas ) ;-)
Right, it looks like unintentional ware, that’s the art
I do a lot of artwork for a Nature Centre. One thing I do use is the clear coat exterior Varithane to coat projects. The SPRAY version which is also water based. This can seal the paint and the box. Since you used a marker, I would not use anything with a thinner base in it that is for oils or laquers etc. Water based products will not run the paint or the pen marks. Always test something similar that you don't want to use or is scrap. I have found this process to work very well! I have even made play cameras with paints and insulation foams, and finished coated with this method. The new exterior products are plastics, and they hold up much better than they used to do! If kid proof, it should also be instrument proof. I have also built , carved and created many different instruments, and used the spray methods for some of the finishes. On wood boxes, don't forget to try out the old fashioned gun stock oil! That simply is the best oil out there and instruments can have anything from soft satin to high polish look when using gun stock oil. Hope this helps:) Cheers!